Oxidized coenzyme Q10, which is a benzoquinone derivative widely distributed in the biological world, is also called vitamin Q because of its vitamin-like function and is an ingredient acting as a nutrient in restoring the cell activity that has been weakened to its healthy condition and rejuvenating the body. On the other hand, reduced coenzyme Q10, which is derived from oxidized coenzyme Q10 by two-electron reduction, is as white crystals as compared with oxidized coenzyme Q10 being as orange-colored crystals. Reduced coenzyme Q10 and oxidized coenzyme Q10 are known to be localized in the mitochondrion, lysosome, Golgi body, microsome, peroxisome, and cell membrane, among others, and involved, as constituents of the electron transport system, in ATP production and activation, in vivo antioxidant activity, and membrane stabilization; they are thus substances indispensable for body function maintenance.
It is known that reduced coenzyme Q10 can be prepared by producing coenzyme Q10 in the conventional manner, for example by synthesis, fermentation, or extraction from natural products, and concentrating a reduced coenzyme Q10-containing eluate fraction resulting from chromatography (JP-A-10-109933). On that occasion, as described in the above-cited publication, the chromatographic concentration may be carried out after reduction of oxidized coenzyme Q10 contained in the reduced coenzyme Q10 with a conventional reducing agent such as sodium borohydride or sodium dithionite (sodium hyposulfite), or reduced coenzyme Q10 may be prepared by reacting the reducing agent mentioned above with an existing highly pure grade of coenzyme Q10 (oxidized form). However, the thus-obtained reduced coenzyme Q10 cannot always be in a highly pure state but tends to occur as a low-purity crystalline, semisolid, or oily product containing such impurities as oxidized coenzyme Q10.
JP-A-57-70834 discloses an example in which reduced coenzyme Q10 was synthesized by dissolving coenzyme Q10 in hexane and adding an aqueous solution of sodium hydrosulfite (sodium hyposulfite) to the solution, followed by stirring. However, sodium dithionite was used as a reducing agent in an amount as large as twice the weight of coenzyme Q10. Thus, such method of synthesis has problems from the economical viewpoint as well as complexity viewpoint of the subsequent purification procedure.
Reduced coenzyme Q10 is readily oxidized to oxidized coenzyme Q10 by molecular oxygen. On a commercial production scale, complete oxygen elimination is very difficult to achieve and, furthermore, fairly long periods of time are required for individual operations, unlike laboratory scale production, so that residual oxygen exerts a great adverse effect. The oxidation in question is directly connected with such yield and quality problems as the formation of hardly eliminable oxidized coenzyme Q10 and adulteration of the product therewith. For obtaining highly pure reduced coenzyme Q10 in the form of crystals, it is important to adequately protect the reduced form from the oxidation mentioned above.